1. Technical Field
The present disclosure relates to image display apparatuses and optical elements.
2. Related Art
In recent years, wearable image display apparatuses such as head mount displays (hereinafter, referred to as HMD) have been attracting attention. Such HMD includes a diffraction light guide plate type which is configured to diffract image light by diffraction optical elements formed in a light incident section and a light emitting section of a light guide plate so that the light is propagated in the light guide plate by total reflection. JP-A-2009-133998 is an example of related art.
It has been proposed to use a plastic material or the like for the light guide plate of the HMD in order to reduce the weight or prevent damage due to impact. Since plastic has a linear expansion coefficient larger than that of a glass, the light guide plate expands or contracts with a temperature change and the grating pitch of the diffraction optical elements formed in the light incident side or the light emitting section on the surface of the light guide plate changes accordingly.
For example, depending on the usage of the HMD, temperature difference occurs between the light incident section and the light emitting section of the light guide plate, which may cause the grating pitch of the diffraction optical elements provided in the light incident section and the light emitting section to be different. As the grating pitch increases, the diffraction angle decreases. Accordingly, when the diffraction angle becomes smaller than the critical angle of the light guide plate, light fails to be propagated in the light guide plate by total reflection, which causes a loss in the light emitted from the light emitting section (image light).